Systems have been proposed for containment and collection of leachate seeping from landfill in response to rainfall. These generally involve a liner of low-permeability material separating the landfill from subjacent strata. The leachate may be collected in apertured conduits in drainage materials above the liner and allowed to flow or be pumped to a disposal location. One problem associated with conventional landfill sites is potential clogging of leachate collection conduits. Another is absence of reliable means for detecting potential failure in leachate collection and also potential failure of the liner. These matters are complicated by the basic nature of the leachate collection system, which is generally formed as a number of distinct subsurface layers buried below a considerable depth of landfill. Failure of the system is commonly detected by checking the quality of groundwater flows in the surrounding area. However, this cannot indicate with any significant precision where a failure has occurred.
Another problem is particularly characteristic of quarrying operations where bedrock is excavated and landfill is thereafter deposited in the site. Bedrock is often below the natural water table of the land surrounding the site. Groundwater tends to seep upwardly through the bedrock and can apply considerable hydrostatic pressure against a liner intended to isolate the landfill from the bedrock. If leachate is collected and removed above the liner, the pressure may not be balanced. This stresses the liner and may contribute to failure of the liner. Also, once bedrock has been removed, stresses in the subjacent geological formations can cause upward heaving of the bedrock. This may rupture the liner.
One system for collecting leachate from waste materials in a landfill site is described in U.S. Pat. No. 4,697,954 issued on Oct. 6, 1987 to Grund. The system comprises a basemented structure with a vaulted ceiling that supports overlying landfill. The structure is constructed of a multiplicity of vault elements supported by hollow piers and arranged to define numerous vertically oriented cavities. Drainage material fills the cavities and a sealing layer is formed over the drainage material. An additional layer of drainage material is deposited on the sealing layer, in contact with the waste material. Locally about each cavity, an upper surface of the sealing layer is inclined to direct liquids seeping from the overlying waste into a vertical conduit embedded in the drainage material of the cavity and extending through the sealing layer. The conduit directs the seepage downwardly into a receptacle in the hollow pier supporting the vault elements defining the cavity. The internal surfaces of the vault elements define additional flow surfaces which direct secondary seepage into the receptacle. The seepage flows from the receptacle through a hose fitted with an inspection glass to a disposal conduit. The Grund system addresses problems of collecting leachate and provides a arrangement permitting inspection of seals, but the cost of erecting such a structure is formidable.
The present invention in its various aspects addresses various problems associated with prior practices regarding landfill sites.